Gas turbines, in particular gas turbines used for power generation, are very sensitive to the quality of inlet air drawn into the combustion chamber of the gas turbine. Particularly, contaminant particulate matter can damage turbine blades and can induce corrosion in the working parts of the turbine. As a consequence, mechanical reliability can decrease, along with operating efficiency.
It is customary in the art of gas turbine manufacture to provide a filter section upstream of the air intake to the combustion chamber of the gas turbine to filter the air drawn in to the required quality. The nature of the filtration will depend on the expected environment in which the turbine is to be operated and particularly on the expected contaminant levels in the air. To cope with a wide variety of contaminant particle sizes while avoiding filters from becoming too-rapidly obstructed, it is known to provide several filtration stages sequentially in the path of the incoming air. Such typically will include coarser pre-filter stages to exclude large-dimension contaminant particles followed by a final filtration stage designed to exclude the very smallest particles, in order to achieve the required design specification for the inlet air to the combustion chamber.
FIG. 1 shows a typical schematic configuration of a conventional gas turbine arrangement, showing major parts only. The gas turbine 1 includes a combustion chamber 3 located between two sets of rotor blades 2. The two sets of rotor blades are connected by a shaft, which causes the two sets of rotor blades to rotate synchronously. Upstream of the combustion chamber are compressor blades which compress the air in the combustion chamber, while downstream of the combustion chamber are turbine blades which extract motive energy from the hot gas leaving the combustion chamber. Into the combustion chamber is injected fuel from a fuel supply 4, the combustion of which fuel elevates the temperature in the combustion chamber to produce the flow of hot gas which drives the turbine blades. Since more power is produced by the combustion of the fuel then is necessary to compress the air in the combustion chamber, useful power may be extracted from the rotation of the shaft while waste heat from the exhaust gases can be further used to produce additional power.
Air is drawn into the combustion chamber by the compressor blades through air transition duct 7, which communicates with the local atmosphere through inlet duct 5, in which is positioned filter assembly 6. Therefore, the air entering the combustion chamber is, by means of suitable configuration of filter assembly 6, maintained within the optimal working specification for the working parts of the turbine.
FIGS. 2 and 3 show examples of configurations of filter assemblies previously known to the Applicant, in diagrammatic longitudinal section.
FIG. 2 shows a two-stage filter arrangement wherein a final filter 9 is combined with a pre-filter 8 to achieve a two-stage filtration of incoming air, which passes through the filter in the direction shown by the arrow. The final filter 9 includes a casing 10 which houses one or more filter elements 24, which are made of a filter medium having a large number of small pores formed therein to trap particulate matter.
The final filter 9 is mounted to the inlet duct by means of mounting frame 11, which itself is placed in the inlet duct so as to occlude the entrance thereto. The mounting frame 11 provides an end seal 11a against which peripheral flange 10a of the casing of the final filter rests, with a sealing gasket 14 interposed therebetween, to seal the casing to the mounting frame. Peripheral flange 10a is urged against gasket 14 by retaining element 12 to securely hold the casing in position in the frame.
A pre-filter 8 is then positioned upstream of the final filter to occlude the aperture in the mounting frame with at least one further pre-filter element 8a, made of a different filter medium, usually having a pore size suitable for trapping particular matter having a larger diameter than that trapped by filter element 24. Pre-filter 8 is mounted directly to mounting frame 11 by means of retaining springs 13, which engage with fixtures on the pre-filter and urge the pre-filter 8 against the mounting frame 11. Therefore, gas drawn in through the inlet duct 5 must pass through both pre-filter 8 and final filter 9 in sequence before reaching the combustion chamber.
FIG. 3 shows an alternative configuration to the arrangement of FIG. 2 in which like parts are shown with identical reference numbers to those in FIG. 2.
The arrangement of FIG. 3 differs in that rather than the pre-filter 8 being urged against mounting frame 11 by retaining springs 13, an adaptor plate is provided having a peripheral flange 15a sandwiched between retaining element 12 and peripheral flange 10a of casing 10, to principal portion 15b of which one or more filter cartridges are engaged by engaging means not shown in the Figure. This latter arrangement is described in detail in co-pending British National patent application number 1016164.4, in the name of the present Applicant.
Shown in FIG. 4 is a third alternative configuration to the arrangement of FIGS. 2 and 3, in which like parts are again shown with identical reference numbers. This configuration is different from that of FIGS. 2 and 3 in that a remote pre-filter holding frame 16 is provided, to which pre-filter 8 is urged by a pre-filter retaining means, not shown. The distance A between the pre-filter holding frame 16 and the filter holding frame 11 is sufficient to permit the filter 9 to be displaced in the upstream flow direction and then removed, e.g. from a side door in the duct in which frames 11 and 16 are located, so as to permit the filter 9 to be changed without removing pre-filter 8.
However, in each of these arrangements, there is a problem that separate mounting arrangements must be made for both the pre-filter and to the main-filter. This leads to complexity in either manufacture or in maintenance, for example when a pre-filter or a main-filter must be changed, generally either both filters will need to be disengaged from a common frame, such as in the examples of FIGS. 2 and 3, or additional space needs to be provided between the pre-filter and the final filter to enable independent maintenance of each, such as in the example of FIG. 4.
There is therefore a need for a pre-filter/main-filter assembly which is easy to maintain while being able to relatively retain the pre-filter and final filter in an operative configuration.